The present invention is directed to xe2x80x9cactive implantable medical devicesxe2x80x9d as such devices are defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, and more particularly to pacemaker, defibrillator and/or cardiovertor devices able to deliver to the heart low energy pulses for the treatment of heartbeat rate disorders. The invention more particularly relates to the adjustment of the amplitude (the voltage level) of the stimulation pulses delivered over time.
The ventricular stimulation level or amplitude is typically a value ranging between 1.5 and 5.0 V (xc2x110%), adjustable in step increments of 0.25 V. This amplitude must be sufficiently high to cause the depolarization of the myocardial cavity; it is necessary, however, to avoid values that are too high in order to spare the lifespan of the battery, because the stimulation energy applied to the myocardium, and therefore the corresponding energy consumption of the device, is proportional to the square of the amplitude (and also to the duration) of the pulse.
Also, the amplitude needed to cause the depolarization is a value that can vary over time. Therefore, it is desirable to be able to reevaluate at regular intervals the stimulation amplitude level needed by operating a test of the threshold of effectiveness of the stimulation, called xe2x80x9ccapture test.xe2x80x9d The stimulation pulse amplitude is then adjusted on the basis of the capture threshold thus measured, typically at a level that is equal to twice the value of the measured threshold, subject to a minimum (typically 1.5 V) and a maximum (typically 5.0 V) amplitude limit.
The patent publication WO-A-93/02741 and its corresponding U.S. Pat. No. 5,411,533 (both assigned to Ela Medical, the assignee hereof) describe an algorithm for automatically testing the ventricular capture threshold. This algorithm is used in commercial pacemaker products sold under the Talent(trademark) brand, available from Ela Mxc3xa9dical, Montrouge, France.
A clinical follow-up of patients equipped with this Talent(trademark) pacemaker device has revealed that, in certain cases, the automatic threshold test algorithm sees its effectiveness reduced because of anomalies occurring at the time of the test. These anomalies may result in an overvaluation of the capture threshold value as compared to the real threshold of the patient. Because any reevaluation of the stimulation amplitude is usually operated every six hours or so, such an excessive level is maintained for at least six hours and, although it is not in itself dangerous, constitutes a source of overconsumption of energy and thus a reduction of the lifespan of the implant.
The aforementioned algorithm for the automatic determination of the capture threshold comprises two specific phases, namely: (1) a preliminary calibration phase, to remove the effect of the polarization of the probe at the heart/electrode interface by determining a reference value; and (2) a measurement phase of the threshold of effectiveness, compared to the reference value previously obtained. As it is easily appreciated, the accuracy of the adjustment is conditioned by the accuracy and the absence of error of the preliminary calibration. The mechanism of a possible calibration error is as follows. The calibration phase comprises at least two measurements of the evoked potential (i.e., a potential of a depolarization consecutive to (i.e., following) a preceding stimulation pulse) for pulses with different amplitudes, for example, pulses of 2 volts and 4 volts. The calibration is regarded as effective if the amplitude of the evoked potential after the calibration stimulation at 2 volts is greater than a certain percentage (typically 25%) of the evoked potential measured with a 4 volt amplitude pulse. But this criterion can be verified for a reason other than an effective capture, which can occur in two circumstances: (1) a very large polarization at the heart/electrode interface, and/or (2) a fusion, i.e., a stimulation intervening in a concomitant way to a spontaneous QRS event occurring at the time of the capture test.
Indeed, the test with 4 volts is always effective (except in the event of a displacement of the probe, but this is an extreme case) and the device will thus measure an evoked potential. On the other hand, the test with 2 volts is ineffective (absence of capture), but the device nevertheless measures a potential which, in fact, is either an elevated polarization potential or a depolarization potential resulting from a fusion that deludes the system and thus results in an erroneous reference.
In the subsequent measurement phase of the capture threshold, the device will systematically measure, for each cycle, the same evoked potential value in spite of the progressive reduction of the stimulation amplitude. Indeed, this measured evoked potential will in fact be only the polarization potential, or the spontaneous depolarization potential resulting from a fusion, in principle always 75% higher than the reference value, and producing an erroneous conclusion of an effective stimulation at each cycle.
U.S. Pat. No. 5,855,594 refers to a method for the determination of the presence of a high voltage of polarization of the electrodes. During a test of capture verification, the device emits pulses with a very low energy below the cardiac capture threshold. The detection of a signal during such stimulations is the sign of a large polarization at the level of the electrodes. However this U.S. patent does not recognize or consider the problem of fusion.
It is, therefore, an object of the present invention to propose a device able to invalidate the calibration and/or the readjustment of the stimulation level in the event of a detection of a positive case of fusion.
To this end, the present invention proposes an active implantable medical device, in particular a pacemaker, defibrillator or cardiovertor, of the general type disclosed in the U.S. Pat. No. 5,855,594 mentioned above, i.e., including stimulation means able to deliver to the heart pulses presenting a predetermined amplitude and duration, and means for adjusting the amplitude of the stimulation pulses, comprising calibrating means, means for delivering a stimulation pulse at null (zero) or at a primarily null voltage (i.e., an effectively null voltage), and means for automatically measuring the ventricular capture threshold, including means for detecting a presence or an absence of a capture consecutive to a stimulation.
According to the invention, the device further includes means for detecting a fusion situation, and means to invalidate the aforementioned adjustment of the stimulation pulses in the event of a detected fusion, these means operating by the delivery of a stimulation pulse at an effectively null voltage, and being able to determine the presence of a fusion in the event of detection of a capture in response to a stimulation at an effectively null voltage.
More particularly, when the means for detecting a fusion situation detects a capture in response to a stimulation at null voltage, these means control the reiteration of the calibrating means, advantageously with a preliminary reduction of the escape interval and/or the atrioventricular delay.
In the alternative or in addition, the means for detecting a fusion situation comprises means able to: (1) deliver a sequence of stimulation pulses at various decreasing successive levels of decreasing voltage and to detect each time the presence or the loss of capture, the stimulation amplitude being adjusted according to the pulse level that produced the loss of the capture, and (2) after application of the last pulse producing a correlative capture, or of the last pulse of the sequence, to apply a pulse at null voltage and validate the adjustment of the threshold amplitude level only when the amplitude of the evoked potential for a stimulation at effectively null voltage is lower than the evoked potential after the aforementioned last stimulation producing a correlative capture.
In all cases, the device very advantageously delivers a counter-stimulation after each delivery of a stimulation at effectively null voltage.